1. Field of the Invention
This invention relates to satellite solar arrays and more specifically to a method for biasing a solar array with respect to its rotational axis to increase the solar array's worst case output power.
2. Description of the Related Art
Satellites orbit the earth to provide point-to-point communications, weather detection, and remote sensing functions. These satellites employ solar arrays, which convert the sun's energy into electrical energy to power the satellite and its subsystems. One type of satellite, commonly known as a 3-axis body stabilized satellite, deploys a pair of solar arrays in a wing configuration along an axis perpendicular to the body of the satellite and its orbital plane.
As the satellite orbits the earth, its attitude is maintained so that the antenna array or sensing devices at the front of the satellite always point towards the earth. This continuously changes the orientation of the satellite, and hence the orientation of the solar array to the sun. As a result, existing satellites rotate the solar arrays around the perpendicular axis to track the daily movement of the sun to maintain the solar array's output power at a high level.
As the earth orbits the sun, the tilt of the earth's spin axis causes the angle or declination of the sun to change. The declination is zero at the autumnal and vernal equinox and attains an absolute maximum of approximately 23.45.degree. at the summer and winter solstices. This seasonal variation also causes the solar array's output power to fluctuate. This seasonal variation in the solar array's output power can be eliminated by providing a second axis of active rotation to track the seasonal movement of the sun. The solar array's minimum output power level is critical because satellites and their payload must be designed to use no more than the minimum output power generated by the solar array. Tracking the seasonal movement, in addition to tracking the daily movement, would maximize the minimum output power of the solar array.
However, 2-axis satellites are not implemented for several reasons. First, the power fluctuations caused by seasonal variations are much less than the fluctuations caused by the orbit of the satellite around the earth. Second, it is not currently cost effective to provide an actuator that is capable of sustaining constant rotation to track the seasonal changes. Furthermore, adding an additional rotation axis and actuator would reduce the satellite's overall reliability and lifetime. Lastly, some fluctuation in the solar array output power is desired. Around the vernal and autumnal equinox the declination of the sun is small enough that the satellite falls in the shadow cast by the earth during its daily orbit. Thus, at equinox the output power must be sufficiently high to not only power the satellite but to recharge its batteries, which then provide power when the satellite is eclipsed.